It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever accepts Islam while holding something, it is his."
Imam al-Shafi'i (may Allah have mercy on him) said: "If his ownership of it is valid for him, then it is his."
Yasin bin Mu'adh al-Zayyat is a Kufan and da'if (weak); Yahya bin Ma'in, al-Bukhari and other hadith masters criticised him. This hadith is only narrated from Ibn Abi Mulaykah from the Prophet (peace and blessings be upon him) as mursal, and from Urwah from the Prophet (peace and blessings be upon him) as mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18259